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Davis, California

Thursday, May 23, 2024

Geneticist harvests secrets of corn

The first thing to greet a visitor to Jeff Ross-Ibarra’s office is the corn. Ross-Ibarra, a plant geneticist at UC Davis, keeps 50 or so dried corncobs on a shelf by the door, arrayed in a neat grid that stretches from wall to wall. But what’s more impressive than the quantity is the variety: there are long cobs, short ones, red, pink, black, white, pointy, smooth, pinecone-shaped and log-shaped ones, a popcorn cob and even some grassy stalks that look like weeds with tiny kernels buried in hard shells. And these stunningly diverse ears all belong to a single species. All of them, even the round black ear and the popcorn, could interbreed.

“Corn is charismatic,” Ross-Ibarra said. “Corn has crazy amounts of diversity. You can see the huge change between the wild ancestor and the modern variety – the wild one looks like a weed, and the modern one has these huge, soft, open kernels on big ears. The dramatic amount of morphological change is very compelling, and I’m interested in figuring out why it’s all there and what it does.”

For some traits, like the big, soft kernels and heavy ears of modern maize, the value to farmers is obvious. But Ross-Ibarra’s lab, using a novel statistical method, has discovered a slew of genes that have clearly been selected for – but no one knows why. These genes seem to have tagged along with more obviously valuable traits.

This method involves comparing the genomes of modern corn to corn’s weed-like wild ancestor, teosinte. Wherever there is a clear drop in diversity over time, there has probably been selection for the trait produced by that gene. Farmers weed out the many types of less-desirable corn and keep planting the varieties that produce “ideal” corn.

Ross-Ibarra’s comparative method is groundbreaking because it tackles the question of selection from the DNA on up. Typically, researchers have first looked at physical traits – big ears, branching of stalks, kernel color – and then worked backwards to isolate the genes responsible. Ross-Ibarra’s approach ignores traits entirely when looking for evidence of selection. This “blind” statistical method has allowed the lab to uncover genes that have been unconsciously selected by farmers, but that no one has ever noticed.

The U.S. Department of Agriculture awarded Ross-Ibarra’s project with a grant for $150,000 in annual support for three years, and as a part of the application, Ross-Ibarra was nominated for and won the prestigious Presidential Early Career Award for Scientists and Engineers.

Why do corn genes matter so much? If the lab’s innovative approach to population genetics isn’t enough, there’s also the fact that corn is a hugely important crop, subsidized by the government to the tune of $2 billion to $10 billion a year since 1995. Michael Pollan, the author of The Omnivore’s Dilemma, wrote that the modern supermarket is basically made of corn: from the feed that the cattle and turkeys eat, to the flour, dyes and sweeteners in packaged foods, to the glossy shine on the magazine covers and even the linoleum and wallboard a building is made of.

Besides being an important industry, corn is a geneticist’s dream. It only has 10 chromosome pairs (compared to the 24 pairs in tomatoes and the 42 in wheat), and its offspring line up in tidy rows to be counted. It’s easy to see at a glance how many kernels carry each parent’s genes.

“I don’t know why more geneticists don’t study corn,” Ross-Ibarra said.

EMILY GOYINS can be reached at science@theaggie.org.


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